In a conventional technique, a DPF (Diesel Particulate Filter) is provided in an exhaust passage to purify an exhaust gas discharged from an engine. With this technique of providing a DPF in the exhaust passage, soot constituted by microparticles (PM) contained in the exhaust gas is collected by the DPF.
When a DPF is provided in the exhaust passage, the soot collected by the DPF accumulates in the DPF. When an amount of soot accumulated in the DPF increases, an exhaust gas pressure loss increases, leading to a reduction in an intake air amount of the engine.
If, at this time, control is performed with respect to EGR for recirculating a part of the exhaust gas to an air supply side, for example, while maintaining target signals (an excess air ratio, an excess oxygen ratio, a supply air flow rate, and so on) of the EGR control at fixed values, an EGR control valve is controlled to a closing side, leading to a reduction in an EGR gas amount, and as a result, an effect of reducing harmful substances in the exhaust gas through EGR decreases.
Therefore, control must be executed precisely in accordance with increases and reductions in the exhaust gas pressure loss.
A technique disclosed in Patent Document 1 is available as an example of control executed in accordance with increases and reductions in the exhaust gas pressure loss. In this technique, a DPF pressure loss condition is determined from a relationship between a DPF front-rear differential pressure and an exhaust gas amount, and when, on the basis of a result of this determination, the pressure loss is larger than a set value, DPF regeneration processing is implemented. Further, when the DPF front-rear differential pressure is not large enough to merit DPF regeneration processing, an upper limit of a fuel injection amount is modified to an increasing side such that a decreasing engine torque is compensated for by an increase in the exhaust gas pressure loss due to soot accumulation in the DPF. When an exhaust gas temperature reaches or exceeds a limit value due to the increase in the exhaust gas pressure loss, on the other hand, the exhaust gas temperature is set at or below the limit value by correcting the fuel injection amount to a decreasing side.
Further, a technique disclosed in Patent Document 2 is available as another example of control executed in accordance with increases and reductions in the exhaust gas pressure loss. In this technique, an intake air amount of the engine is limited when an exhaust manifold pressure exceeds a limit pressure, and the exhaust manifold pressure is calculated on the basis of a DPF pressure loss and a DPF differential pressure. Further, the intake air amount is limited on the basis of a comparison between the DPF differential pressure and a limit value. Furthermore, limits are applied to both the engine intake air amount and the fuel injection amount.
Patent Document 1: Japanese Patent No. 3856118
Patent Document 2: Japanese Patent Application Publication No. 2007-40269
In the technique disclosed in Patent Document 1, however, a regeneration start determination value is determined from the exhaust gas flow rate and the filter pressure loss, and then compared with an actual filter pressure loss to determine whether to switch correction control ON/OFF. However, the correction control ON/OFF evaluation is preferably not performed as is because between exhaust gas flow rate variation and filter pressure loss variation, sensor response differences and pressure response delays relative to the flow rate and variation may occur. Further, increases and reductions in the pressure loss are evaluated simply from the pressure loss of the filter portion, and pressure loss in pipe parts to the front and rear of the filter is not evaluated. It cannot therefore be said that an accurate evaluation is performed with respect to the pressure loss.
Further, in the technique disclosed in Patent Document 2, the exhaust gas flow rate is not taken into account, and therefore the obtained exhaust manifold pressure is greatly affected by engine operating conditions. In this case, the obtained exhaust manifold pressure can be used without problems to determine the intake air amount (a turbocharging pressure) and whether to switch fuel injection amount limitation ON or OFF, but cannot easily be used in control for correcting an injection timing, an injection pressure, and so on.
Hence, from the techniques relating to Patent Documents 1 and 2, it can be said that since the exhaust gas pressure loss varies from moment to moment in accordance with the engine operating conditions, it is difficult to use a measured exhaust gas pressure loss directly in control.